Rheology and fabric in the continental lithospheric mantle from naturally deformed peridotites

Lower crust and upper mantle xenoliths, which come to the surface through volcanic eruptions, give us a direct window into deep lithospheric deformation. By using techniques such as electron backscatter diffraction (EBSD), we can use xenoliths to place natural con- straints on the extrapolation of laboratory flow laws to Earth and inform geodynamic and geophysical models that rely heavily on knowledge of rheological structure. In Chapter 2, we use xenoliths from the tectonically active Mojave region of southern California to inform us of the nature of the lithosphere in this complex region. The results gleaned from xenoliths demonstrate that the mantle lid beneath the eastern Mojave is deforming by dislocation creep at relatively low stresses and fast strain rates, with an associated average viscosity of ~10¹⁹ Pa*s. The xenoliths also record a wide range of microstructures indicative of substantial strain localization at Moho depths. Finally, a study of the crystallographic or lattice preferred orientation (LPO) of crustal and mantle xenoliths reveal (1) that the mantle lid preserves both modern and fossilized anisotropy, and (2) crustal and mantle seismic fast axes may be anticorrelated across the Moho, even when both layers are kinematically coupled. In Chapter 3, we use these Mojave xenoliths in addition to several peridotites (a total of 65 samples) from a variety of geologic settings to study the broader relationship between olivine LPO and deformation conditions. Of particular interest are water content and stress, which past experiments have found strongly influence olivine LPO. Data from these samples, combined with an extensive literature compilation, reveal the extremely complicated nature of olivine LPO types and their link to deformation conditions. The results suggest that factors such as deformation geometry and history play a greater role in determining olivine LPO type than stress magnitude, temperature, and water content -- factors thought to be most influential based on experiments. In Chapter 4, we utilize the dataset of Chapter 3 to study the effect of orthopyroxene on bulk anisotropy. Among those samples with both olivine and orthopyroxene LPO, seven types of orthopyroxene LPO are identified. For each sample, measures of anisotropy are calculated from LPO data weighted for a range of hypothetical olivine and orthopyroxene modal percentages. Results reveal that for typical mantle peridotite, orthopyroxene LPO decreases anisotropy but does not affect the orientation of seismic fast directions.